This invention relates to a digital signal processing circuit for processing analog signals such as audio signals and video signals in the form of digital signals and, more particularly, to a digital signal processing circuit capable of preventing clipping distortion from occurring in an output of the circuit when the circuit has a gain.
A signal system for digitally processing analog signals such as audio signals or video signals is generally constructed as shown in FIG. 2. In the signal system, an analog signal to be processed digitally is first converted to a digital signal by an analog-to-digital converter 10 and thereafter is processed as a digital signal by a digital signal processing section 12. The signal which has thus been processed as a digital signal is subsequently restored to an analog signal by a digital-to-analog converter 14 and delivered out of the system. The analog-to-digital converter 10 is disposed on the side of a playback device when an analog reproduced output such as one from an analog record is to be processed whereas it is disposed on the side of a recording device when a digital reproduced output such as one from a Compact Disc is to be processed (i.e., the reproduced output is applied directly to the digital signal processing section 12).
The digital signal processing section 12 is constructed specifically as a digital filter, digital graphic equalizer, digital effector or digital reverberator or the like digital device. These digital devices generally have a gain in themselves. Hence, overflow tends to occur in the process of signal processing when an input of a large level has been applied to the device with resulting occurrence of clipping distortion in an output waveform.
Assume, for example, that the digital signal processing section 12 is constructed of a digital graphic equalizer having amount of boost of +6 dB in a case where the analog-to-digital converter 10 and digital signal processing section 12 both consist of 16 bits and the level of an analog input is so adjusted that the conversion output of the analog-to-digital converter becomes data of full bits (i.e., value immediately before overflowing) with respect to an analog input of 0 dB (maximum reference level). In this case, if the graphic equalizer is boosted during application of an analog input signal of 0 dB, this graphic equalizer 12 naturally overflows with a result that clipping occurs in the output waveform. Accordingly, for providing a head margin in the digital signal processing section 12, the maximum acceptable input of the analog-to-digital converter 10 during boost has to be reduced to -6 dB.
As described above, in the system in which boost is carried out, the level of analog input must be reduced by the amount of boost before the analog input is applied to the analog-to-digital converter 10.
If the accuracy of the analog-to-digital converter 10 is satisfactory, the reduction in the level of the analog input will result only in increase in quantizing error. According to measurement conducted by the inventor of this invention, however, currently available analog-to-digital converters for consumer uses have a rather inferior linearity and actually measured accuracy of conversion of these converters are much worse than was expected. In an analog-to-digital converter of 16 bits, for example, it has been found that the accuracy of conversion is only in the order of 14 bits and the less significan 2 bits are reduced to a conversion error. For this reason, the greater the extent of reduction of the level of analog input, the worse is the condition under which the analog-to-digital converter 10 is used and so is signal-to-noise-ratio.
Besides, if the above described method of reducing the level of anlog input is to be applied to a case where a digital output such as one of a Compact Disc is applied directly to the digital signal processing section 12, an analog input to be recorded in the Compact Disc must also be reduced. This will necessitate alteration in the process of manufacturing of the Compact Disc and hence is not practicable.
It is, therefore, an object of the present invention to provide a digital signal processing circuit which has eliminated the above described disadvantages of the prior art circuits. More specifically, it is an object of the invention to provide a digital signal processing circuit capable of preventing overflow in digital signal processing without reducing the level of an analog input in a digital signal processing section having a gain whereby the adverse influence of conversion error in the analog-to-digital conversion is reduced and deterioration in the signal-to-noise ratio is prevented.